The Trigger: Bursts of Ultraviolet Light and Xrays from Solar Flares

Just 8 minutes after a solar flare occurs on the Sun a blast of ultraviolet and xray radiation hits the dayside of the Earth. This high energy radiation is absorbed by atmospheric particles raising them to excited states and knocking electrons free in the process of photoionization. The low altitude D- and E-region ionospheric layers immediately increase in density over the entire dayside.

The Effects on Radio Waves

Short wave radio waves (in the HF range) are absorbed by the increased particles in the low altitude ionosphere causing a complete black out of radio communications. This is called a Short Wave Fadeout.

How Long Does a Short Wave Fadeout Last?

Short wave fadeouts last for a few minutes to a few hours and are the most severe in the equatorial regions where the Sun is most directly overhead.